1. (Field of the Invention)
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system for internal combustion engines in which an alcohol blended fuel is used.
2. (Prior Art)
Recently, internal combustion engines have been intensively studied and developed, which use a fuel containing alcohol such as methanol, in order to reduce the amount of noxious exhaust gas components such as NOx as well as to save the cost of fuel used.
Conventionally, in an internal combustion engine of this kind, the air-fuel ratio of an air-fuel mixture supplied to the engine is controlled to a stoichiometric air-fuel ratio in response to the concentration of alcohol present in the fuel, detected by an alcohol concentration sensor mounted in the engine, as proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 1-244133.
However, according to the conventional air-fuel ratio control method, if output from the alcohol concentration sensor (i.e. the detected value of alcohol concentration) undergoes a change due to spark noise, drift, etc. to deviate from the actual value of alcohol concentration, especially at the start of the engine, the air-fuel ratio of the mixture is controlled to a value deviated from the stoichiometric value in response to the output from the alcohol concentration sensor, which results in degraded exhaust emission characteristics.
Further, a simple way to control the air-fuel ratio of a mixture containing alcohol would be to detect the concentration of alcohol present in the fuel by an alcohol concentration sensor within a predetermined time period after the start of the engine to determine an alcohol concentration-dependent correction coefficient for use in calculation of the amount of fuel to be supplied to the engine.
However, according to this method, since the alcohol concentration-dependent correction coefficient is determined soley based on values of the concentration of alcohol in the fuel detected within the predetermined time period after the start of the engine, there is a fear that the air-fuel ratio is improperly controlled, since the concentration of alcohol in the fuel can vary due to refueling or depending on the mixed state of alcohol and gasoline in the fuel tank so that the determined value of the alcohol concentration-dependent correction coefficient becomes different from a value thereof corresponding to the actual alcohol concentration.
Further, the alcohol concentration sensor is usually mounted in the wall of a fuel supply pipe of the engine. As a result, if the engine is restarted when it is hot, hot fuel which has been heated by the hot fuel supply pipe of the engine is first presented to the alcohol concentration sensor, and then cold fuel newly flowing out of the fuel tank is presented to same, which can result in that the output level of the alcohol concentration sensor temporarily becomes deviated from a proper output level due to temperature-dependent characteristics of the alcohol concentration sensor. Consequently, the detected value of alcohol concentration becomes deviated from a correct value, and accordingly the alcohol concentration-dependent correction coefficient determined based on the deviated detected value of alcohol concentration does not correspond to the actual value of the alcohol concentration. Therefore, the engine is not properly controlled, resulting in degraded exhaust emission characteristics.